It is generally believed that there are a number of animal diseases in which the disease promoting organism causes an interaction between macrophages and T-cells (lymphocytes) to induce the production of tissue necrosis factor (TNF-.alpha.) which can have serious adverse effects upon the animal. Among such diseases are cerebral malaria, endotoxin shock, toxic shock syndrome, streptococcal toxic shock syndrome, respiratory failure caused by mycoplasma super antigen, staphylococcus enterotoxin diseases, and diseases caused by parasitic super antigens or viral super antigens.
The human disease malaria, particularly malaria caused by Plasmodium falciparum, remains a serious medical challenge. It afflicts over 200 million people and in Africa alone is responsible for over one million deaths among children each year. Cerebral malaria is a most severe and frequently fatal consequence of falciparum malaria in the young and non-immune. Cerebral malaria causes acute symptoms including leaky blood vessels in the brain and cerebral edema. Antimalarial chemotherapy initiated at the time of cerebral symptoms often fails to alter the fatal outcome, and mortality even with the best current therapies ranges from 10 to 50%. The pathological processes responsible for the cerebral manifestations are not well understood. Cerebral tissue obtained post-mortem reveals grossly congested cerebral vessels, occluded with infected red blood cells adherent to the endothelium, which leads to the subsequent encephalopathy secondary to cerebral anoxia.
An immunopathological component to the cerebral manifestations has been suggested from several studies which have implicated a role for tumor necrosis factor (TNF). The malaria disease causing agent causes activation of T-cells (CD4 and possibly CD8) and macrophages which release the cytokines TNF-.alpha., IL-1 and IL-6 which lead to increased vascular permeability and cerebral edema. In the murine model of cerebral malaria, the immunopathological role of TNF has been most thoroughly studied. It has been observed that cerebral pathology and mortality have been abrogated by concurrent treatment with antibodies against cytokines associated with cell activation and TNF production and specifically with anti-TNF antibodies(1,2,3). It is important to note that some of the pathological processes in the murine model are similar to those in human cerebral malaria, e.g., the underlying mechanism of vascular occlusion is closely paralleled(4,5,6). Typically in the murine model, CBA/ca mice are infected with Plasmodium berghei ANKA strain and within 6-15 days 50% or more will expire with cerebral manifestations, primarily ataxia, paralysis and convulsions. Histopathology reveals grossly occluded cerebral vasculature(7).
In keeping with the importance of TNF in human cerebral malaria pathology, two case reports have described remarkable recovery from cerebral malaria when antimalarial therapy is combined with phosphodiesterase inhibitors that inhibit TNF synthesis(8,9). Amelioration of cerebral manifestations and mortality has also been accomplished in the murine CM model with the methylxanthine pentoxifylline and the prostacyclin analog Illoprost without antimalarial treatment(10,11).
In addition to cerebral malaria, gram-negative septic shock, which is characterized by a high mortality rate and is responsible for hundreds of thousands of deaths annually, appears to involve the production of excess TNF-.alpha. which results from a cascade of events triggered by the action of bacterial endotoxin which is a lipopolysaccharide (LPS).
Of particular importance concerning the lethal effects of LPS is the observation that nanogram quantities of LPS can induce the release of mediators such as tumor necrosis factor-.alpha., (TNF-.alpha.) interleukin-1 (IL-1), and interleukin-6 (IL-6). The release of mediators such as tumor necrosis factor-.alpha. (TNF), interleukin-1 (IL-1), and interleukin-6 (IL-6) is thought to produce the toxicity associated with endotoxemia. However, despite the high mortality rate of endotoxic shock, relatively little is known about the biochemical and cellular mechanisms involved in LPS-induced events, e.g., TNF and IL-1 release, although an amplification system is suggested given that nanogram quantities of LPS can produce severe toxicity in animals. Most amplification pathways involve receptors and various enzyme cascades, thus allowing for several points of antagonism within the system. Although certain steps in LPS action are known, such as the stimulation of phosphoinositide hydrolysis, a transient increase in intracellular Ca.sup.++ levels, and the activation of protein kinase C and phospholipase A.sub.2, the lack of specific information on the cellular mechanisms involved in LPS action has impeded the development of therapeutic agents for preventing endotoxin shock.
Macrophages are particularly important cells in LPS-mediated TNF-.alpha., IL-1, and IL-6 release. Although a detailed understanding of the cellular and biochemical processes through which LPS activates macrophages is unknown several lines of indirect evidence suggest that G-proteins might be involved in LPS action, which would be consistent with a receptor-linked cellular amplification pathway. Our earlier finding that LPS stimulates a macrophage membrane-associated GTPase, an activity that is a hallmark of G-protein involvement, further supports a role for G-proteins in endotoxicity.
Obviously, it would be advantageous to have agents and methods to prevent disease causing agents, such as the malaria parasite and LPS, from producing tumor necrosis factor (TNF).